This invention relates to densified composite articles, such as those made of ceramic-metal compositions, and methods of forming such articles. More particularly, the invention relates to such composite articles that include hollows or cavities of complex geometry and methods of formation whereby machining of internal surface is not required.
Making composite articles of ceramic-metal, metal-metal or ceramic-glass materials often involves formation of a porous compact of the materials followed by a densification process such as, for example, sintering or hot isostatic pressing. Densification processes typically result in considerable shrinkage of the porous compact as it increases from 50-60 volume percent to 80-100 volume percent of complete or theoretical density. Densification processes also tend to distort article dimensions and result in many reject pieces that do not have the desired finished or net shape of the article.
The difficulty of achieving a dimensionally predictable and acceptable finished composite article of desired net shape is particularly acute for articles that must include an internal surface or cavity or hollow portion, particularly if the cavity comprises complex internal geometry. Such articles might comprise, for example, a tube, a tube shape having a variable internal diameter or a hollow ball.
In the past, cavities in composite articles often have been made by boring out a capillary or passageway in the sintered article, followed by machining where variable diameters or other complex surfaces are required. Articles having completely enclosed cavities or hollows are typically made by forming the articles in separate sections followed by joining or welding the sections together to form the whole.
These prior art techniques are expensive and often technically difficult, particularly where internal working space is limited. Where the materials of which the articles are made are very hard, such as for many ceramics, even simple borings may become exceedingly difficult and expensive.
As an alternative to sintering or hot pressing, ceramic-metal composites may be densified to form by infiltrating the metal into a porous ceramic compact. Thus, Stibbs et al. in U.S. Pat. No. 3,749,571 describes infiltrating silicon into a boron carbide compact. While a density of 99 percent of theoretical density was said to be achieved, a necessary sintering step still results in shrinkage of the article. Gazza et al. in U.S. Pat. No. 3,864,154 describe making a solid composite, such as a simple disk, of various ceramicmetal materials by surrounding a ceramic compact with powdered metal followed by heating until molten metal impregnates the ceramic skelton. Landingham in U.S. Pat. No. 3,718,441 achieves a degree of densification of a beryllium oxide compact of simple cylindrical geometry by reducing oxide films present on metal powders that are said to prevent metal from wetting the ceramic during sintering operations. Excess metal must then be machined off.
The focus of this earlier work was simply to achieve densification. The articles of interest were not complexly shaped articles having internal cavities. Thus, neither internal shrinkage nor internal machining to net shape was of significant concern.
Where more reactive combinations of materials, such as B.sub.4 C and A1, are of interest and as infiltration process is employed, one often must balance the infiltration process with kinetics of potential chemical reactions that form ceramic phases that may interfere with densification by blocking infiltration channels in the porous compact. In order to achieve infiltration and, consequently, desired densification, Pyzik et al. in U.S. Pat. No. 4,707,770 reduce reaction rates of a B.sub.4 C-A1 system by thermally treating B.sub.4 C at about 1800.degree. C. prior to metal infiltration of the porous compact. Halverson et al in U.S. Pat. No. 4,718,941 employ a chemical treatment over a prolonged time period to allow infiltration. These material treatment techniques, of course, add cost to the composite article. Most of these densification techniques also involve later sintering steps that cause significant shrinkage of the final products. Consequently, heretofore, infiltration has achieved no advantage over hot pressing in producing net shape composities.
The properites of composites have improved as the ability to produce near fully densified and pore-free articles has progressed. It is now possible to realize the potentials of combining the properties of the materials, such as ceramics and metals. It is now desirable to focus upon producing densified composite articles of a geometric complexity that meet the functional demands of suitable applications that are now evident from the improved properties of the composite materials. There is a need to produce composite articles that are dimensionally precise yet include complex geometry, such as internal cavities and the like. It is desirable to produce such articles in a fully densified, "net shape" form, that is, without significant shrinkage and, thus, completely finished without the need for further shaping, such as by machining.